Commonly manufactured glass formulations often have softening points less than 1800° F., and glass formulations used for wool fiber production often have softening points in the range of 1200-1600° F. Glasses with higher softening points can be expensive to manufacture due to the high energy input, low throughput, and specialty equipment required. High temperature applications may require exposure of wool glass products to temperatures in excess of 1800° F., which would cause the wool glass products to fail through softening.
Glass compositions formed from basalt and basalt-like rocks, melted and formed into fibers, often have relatively good working ranges for fiberization and the resulting fibers often possess good fire resistance properties through crystallization mechanisms. In particular, high temperature wool glasses can be manufactured from basaltic rocks with no additions of other components. Under high temperature conditions, basalt fiber readily crystallizes instead of softening, thereby affording sufficient high temperature resistance. The primary drawback of basalt compositions is poor biosolubility, typically biodissolution rates of less than 5 ng/cm2/hr. In wool glass form, pure basalt glass fiber is not acceptable because of the potential health risk associate with the low biodissolution rate.